Getting rid of colours in textures
The RGB Curves node sounds like what you are looking for. It functions almost exactly the same way Photoshop's curves command does, so it is very useful for pushing light and/or dark values.
See the image below for a quick example, where I left lighter colors (on the right) more or less unchanged, but drastically darkened anything below about the halfway threshold.

The RGB Curves node is also handy for tweaking the exposure levels of a background photo/HDRI without having to modify the original file.